The present invention relates to an ink jet recording apparatus in which liquid ink is made into droplets, that are flown onto a recording sheet, so as to record an image, and more particularly, to an ink jet recording apparatus in which ink droplets are ejected and flown onto a recording sheet by the pressure of an ultrasonic wave beam radiated from a piezoelectric element or elements.
An apparatus of recording an image with image dots formed by making liquid ink into droplets and flying them on a recording sheet is practically used as an ink jet printer. The ink jet printer entails advantages that noise is low as compared to other recording mode printing apparatus, and the process of development, fixation or the like are unnecessary. Thus, the ink jet printer draws much attention as a plain paper recording technique. Up to the present day, a great number of ink jet printer modes have been proposed. In particular, the mode of emitting ink droplets by the pressure of vapor generated by heat of a heat generator, discussed in Jpn. Pat. Appln. KOKOKU Publications No. 56-9429 and No. 61-59911, and the mode of emitting ink droplets by a pressure pulse made by the displacement of a piezoelectric member, discussed in Jpn. Pat. Appln. KOKOKU Publication No. 53-12138, are typical examples of the ink jet printer.
However, with the above-described modes, local concentration of ink is likely to occur due to the evaporation or volatilization of the solvent used. In addition, individual nozzles each corresponding to a respective resolution are very slender, and thus the nozzles may readily be plugged. Particularly, in the mode of utilizing the vapor pressure, the adhesion of an undissolved matter created by the thermal or chemical reaction with the ink easily causes the plugging up of a nozzle, whereas in the mode of utilizing the pressure generated by the displacement of a piezoelectric member, the complex structure including the ink passage even more readily causes the plugging up of the nozzle. In a serial head which employs several tens to a hundred and several tens of nozzles, the frequency of the occurrence of the plugging up can be suppressed; however in the case of a line head which requires several thousand nozzles, the plugging up occurs very frequently, which creates a serious drawback of low reliability. Furthermore, these modes are not suited for improving resolution.
To overcome the above-described drawbacks, there has been proposed a mode of utilizing an ultrasonic wave in which ink droplets are emitted from the surface of liquid ink with use of the pressure of an ultrasonic beam generated from the thin film piezoelectric member (see, for example, IBM TDB, vol. 16, No. 4, page 1168 (1973-10), Jpn. Pat. Appln. KOKAI Publication No. 63-162253). This mode is of a so-called nozzleless type which does not require a nozzle for each and individual dot, or a separation wall between ink passages. Therefore, it is free from the problem entailed in the line head, that is, the plugging up or the restoration of the nozzle from the plugging up. Further, with the ultrasonic wave mode, it is possible to emit an ink droplet of a very small diameter, in a stable manner, and therefore a high resolution can be achieved. However, the ultrasonic wave mode has a low ink droplet flight rate, and as a result, the image recording rate cannot be improved.
In addition, in a typical structure of the head of the conventional ultrasonic wave mode ink jet recording apparatus, acoustic lenses which constitute ultrasonic wave focusing means, especially, Fresnel lens, are made to serve as a supporting members for an ink holding chamber for reserving and holding ink liquid therein. Therefore, in order to improve the mechanical strength of the supporting members, the Fresnel lens is made to have a sufficient thickness as compared to that of the piezoelectric member, which is equal to or larger than the depth of the ink liquid. With this structure, an ultrasonic wave radiated from a piezoelectric element attenuates and/or scatters while it is propagated within the Fresnel lens due to its thickness, and therefore it is very difficult to radiate the ultrasonic wave into the ink liquid at high efficiency. Particularly, in the case where an ultrasonic wave having a high frequency is radiated in order to emit an ink droplet of a small diameter, the attenuation or scattering of the ultrasonic wave within the Fresnel lens causes a great influence on the performance of the apparatus.
As described above, with the conventional mode or structure, it is very difficult to emit or fly ink droplets at high efficiency, and for the high efficiency, it is conventionally required to apply an excessive voltage to the piezoelectric element, and prolong the time for applying a voltage. As a result, the conventional technique entails the problems of an increased consumption power and a low image recording speed.